In addition, various data services are supported, with user bit rates up to 9600 bps. Specially equipped GSM terminals can connect with PSTN, ISDN, Packet Switched and Circuit Switched Public Data Networks, through several possible methods, using synchronous or asynchronous transmission. Also supported are Group 3 facsimile service, videotex, and teletex. Other GSM services include a cell broadcast service, where messages such as traffic reports, are broadcast to users in particular cells.
A service unique to GSM, the Short Message Service, allows users to send and receive point-to-point alphanumeric messages up to a few tens of bytes. It is similar to paging services, but much more comprehensive, allowing bi-directional messages, store-and-forward delivery, and acknowledgement of successful delivery.
Supplementary services enhance the set of basic teleservices. In the Phase I specifications, supplementary services include variations of call forwarding and call barring, such as Call Forward on Busy or Barring of Outgoing International Calls. Many more supplementary services, including multiparty calls, advice of charge, call waiting, and calling line identification presentation will be offered in the Phase 2 specifications.
The Base Station Controller (BSC) manages the radio resources for one or more BTSs, across the Abis interface. It manages the radio interface channels (setup, teardown, frequency hopping, etc.) as well as handovers.
The Home Location Register (HLR) contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the subscriber. The location assists in routing incoming calls to the mobile, and is typically the SS7 address of the visited MSC. There is logically one HLR per GSM network, although it may be implemented as a distributed database.
The Visitor Location Register contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. Although the VLR can be implemented as an independent unit, to date all manufacturers of switching equipment implement the VLR together with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR. The proximity of the VLR information to the MSC speeds up access to information that the MSC requires during a call.
The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile equipment is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, used for authentication and ciphering on the radio channel.
There is a basic distinction between dedicated and idle modes, that arises from on-demand channel allocation due to spectrum scarcity. Dedicated, or traffic, channels provide a bi-directional point-to-point transmission link to a mobile subscriber. Full-rate Traffic Channels (TCH/F) and half-rate Traffic Channels (TCH/H) are allocated together with a low bit-rate Slow Associated Control Channel (SACCH), which typically transmits measurements needed for handover decisions. There are also eighth-rate Traffic Channels, also called Stand-alone Dedicated Control Channels (SDCCH), which are used primarily for transmitting location updating information. In addition, a TCH slot can be pre-empted for signalling, in which case it is called a Fast Associated Control Channel (FACCH), which can be either full-rate or half-rate. TCHs are defined within a 26-frame multiframe.
Common channels can be accessed both by idle mode mobiles, in order to change to dedicated mode, and by dedicated mode mobiles, to monitor surrounding base stations for handover information. The common channels, which are defined within a 51-frame multiframe, include:
This 13 kbps digital stream (260 bits every 20 ms) has forward error correction added by a convolutional encoder. The gross bit rate after channel coding is 22.8 kbps (or 456 bits every 20 ms). These 456 bits are divided into 8 57-bit blocks, and the result is interleaved amongst eight successive time slot bursts for protection against bursty transmission errors.
Each time slot burst is 156.25 bits and contains two 57-bit blocks, and a 26-bit training sequence used for equalization. A burst is transmitted in 0.577 ms for a total bit rate of 270.8 kbps, and is modulated using Gaussian Minimum Shift Keying (GMSK) onto the 200 kHz carrier frequency. The 26-bit training sequence is of a known pattern that is compared with the received pattern in the hope of being able to reconstruct the rest of the original signal. Forward error control and equalization contribute to the robustness of GSM radio signals against interference and multipath fading.
The digital TDMA nature of the signal allows several processes intended to improve transmission quality, increase the mobile's battery life, and improve spectrum efficiency. These include discontinuous transmission, frequency hopping and discontinuous reception when monitoring the paging channel. Another feature used by GSM is power control, which attempts to minimize the radio transmission power of the mobiles and the BTS, and thus minimize the amount of co-channel interference generated.
The RR functional layer is responsible for providing a reliable radio link between the mobile station and the network infrastructure. This includes the establishment and allocation of radio channels on the Um interface, as well as the establishment of A interface links to the MSC. The handover procedures, an essential element of cellular systems, is managed at this layer, which involves the mobile station, the base station subsystem, and, to a lesser degree, the MSC. Several protocols are used between the different network elements to provide RR functionality.
The MM functinal layer assumes a reliable RR-connection, and is responsible for location management and security. Location management involves the procedures and signalling for location updating, so that the mobile's current location is stored at the HLR, allowing incoming calls to be properly routed. Security involves the authentication of the mobile, to prevent unauthorised access to the network, as well as the encryption of all radio link traffic. The protocols in the MM layer involve the SIM, MSC, VLR, and the HLR, as well as the AuC (which is closely tied with the HLR). The machines in the network subsystem exchange signalling information through the Mobile Application Part (MAP), which is built on top of SS7.
The CM functional layer is divided into three sublayers. The Call Control (CC) sublayer manages call routing, establishment, maintenance, and release, and is closely related to ISDN call control. The idea is for CC to be as independent as possible from the underlying specifics of the mobile network. Another sublayer is Supplementary Services, which manages the implementation of the various supplementary services, and also allows users to access and modify their service subscription. The final sublayer is the Short Message Service layer, which handles the routing and delivery of short messages, both from and to the mobile subscriber.
M. Mouly and M.-B. Pautet, GSM Protocol Architecture: Radio Sub-system Signalling, IEEE 41st Vehicular Technology Conference, 1991.